Hexavalent chromium (chromate) is a potent human carcinogen that is present in the workplace of about 500,000 U.S. workers and at several hundred Superfund sites. Drinking water contamination and particulate ambient pollution are two other significant sources of human exposure to Cr-6. Assessment of individual doses of Cr-6 would greatly improve understanding of risks associated with environmental contamination and alleviate public concerns about the extent of the actual human exposures. Because Cr-6 is reduced in human body to Cr-3, measurements of total Cr levels in biological specimens are unable to differentiate between exposures to carcinogenic Cr-6 and ubiquitous forms of nontoxic Cr-3. Thus, the main approach to the estimation of human exposure to toxic Cr compounds should be based on the analysis of specific biological changes caused by Cr-6. Cr-DNA adducts are a unique form of DMA modifications produced by Cr-6 in human cells and therefore, they offer a potential to serve as highly specific indicators of individual doses of Cr-6. Our recent findings demonstrated that cellular reduction of Cr-6 by vitamin C was a principal route to high mutagenicity and genotoxicity of this metal. These results led to the uncertainty in the development of Cr-6 biomarkers as we poorly understand which Cr-DNA adduct(s) are most important toxicologically in cells containing vitamin C. The main goal of this project is to identify the most potent mutagenic and genotoxic Cr-DNA adducts arising from reductive metabolism of Cr-6 by cellular ascorbate. Three Specific Aims will examine (i) the formation of mutagenic Cr-DNA adducts in human lung cells, (ii) the relationship between mutagenicity and genotoxicity of bulky Cr-DNA adducts, and (iii) mechanisms of hypersensitivity of ascorbatesupplemented cells to Cr-6 compounds. The results of proposed studies are expected to provide the basis for the development of mechanistically important, specific biodosimeters of human exposure to different chemical and physical forms of hexavalent chromium.